fix(documents): surface timeline density fetch failures via console.warn (#385)

Previously a 5xx, network blip, or JSON parse error all collapsed
into the same silent "no buckets" rendering. The widget still
degrades gracefully — failure should not block the document list —
but operators and Sentry now see the failure in browser devtools
instead of having to reverse-engineer a missing chart.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/document/timeline.spec.ts

@@ -285,6 +285,29 @@ describe('fetchDensity', () => {
 
 		expect(result).toEqual({ density: [], minDate: null, maxDate: null });
 	});
+
+	it('emits console.warn with the status when the response is non-ok', async () => {
+		const fetch = vi.fn().mockResolvedValue({ ok: false, status: 503 });
+		const warn = vi.spyOn(console, 'warn').mockImplementation(() => {});
+
+		await fetchDensity(fetch, null, true);
+
+		expect(warn).toHaveBeenCalledTimes(1);
+		expect(warn.mock.calls[0][0]).toContain('503');
+		warn.mockRestore();
+	});
+
+	it('emits console.warn with the caught error when fetch rejects', async () => {
+		const error = new TypeError('Network down');
+		const fetch = vi.fn().mockRejectedValue(error);
+		const warn = vi.spyOn(console, 'warn').mockImplementation(() => {});
+
+		await fetchDensity(fetch, null, true);
+
+		expect(warn).toHaveBeenCalledTimes(1);
+		expect(warn.mock.calls[0]).toContain(error);
+		warn.mockRestore();
+	});
 });
 
 describe('tickIndicesFor', () => {

frontend/src/lib/document/timeline.ts

@@ -208,14 +208,18 @@ export async function fetchDensity(
 
 	try {
 		const response = await fetch(buildDensityUrl(filters));
-		if (!response.ok) return EMPTY;
+		if (!response.ok) {
+			console.warn(`[timeline] density fetch responded with ${response.status}`);
+			return EMPTY;
+		}
 		const body = (await response.json()) as DocumentDensityResult;
 		return {
 			density: body.buckets,
 			minDate: body.minDate ?? null,
 			maxDate: body.maxDate ?? null
 		};
-	} catch {
+	} catch (error) {
+		console.warn('[timeline] density fetch failed', error);
 		return EMPTY;
 	}
 }